Proper alignment of the eyes is essential for clear, single vision. Misalignment during early development can lead to amblyopia, a permanent visual impairment. Eye alignment during gaze changes is determined both by the anatomical organization of extraocular muscles and by the coordinated, visually-guided control of those muscles. The detection and correction of alignment errors from binocular comparison of retinal images is referred to as Disparity Vergence, and has both reflexive and voluntary aspects. The reflexive component of disparity vergence corrects horizontal, vertical and cyclotorsional errors of alignment, while voluntary control is restricted to horizontal vergence. This project is concerned with the visual information processing that provides the basis for reflexive disparity vergence, as revealed by vertical vergence responses. Previous work by the Principal Investigator has shown that the vertical vergence controller can extract vertical disparity signals from dynamic random dot stereograms, but that vertical vergence is not influenced by visual attention or subject effort and often occurs without conscious awareness. These movements thus reflect visual processes that are binocular, most probably cortical, but pre-conscious. The experiments in this project provide a way to study processing at an intermediate stage of the visual system. These processes are central to the control of eye alignment, but cannot be studied with conventional psychophysical techniques because they do not necessarily contribute to visual perception. An eye tracking device is used to detect small changes in eye alignment made in response to imposed vertical image misalignment, allowing for measurement of the vergence system's sensitivity to a variety of image parameters. Proposed experiments will determine the role of contrast, spatial and temporal frequency, and visual feature type in the control of reflexive vergence eye movements. Measurements in subjects with abnormal binocularity will follow up on preliminary evidence that reflex vergence is intact in some cases of stereoblindness. Comparison to results from conventional psychophysical sensitivity measures will highlight differences between early (pre-conscious) and later (perceptual) visual processes. The long-term benefit of this research will be improvements in the diagnosis and treatment of binocular visual disorders of eye alignment and depth perception.